The present invention broadly relates to digital data sequences and more specifically pertains to a new and improved method of, and apparatus for, encoding or enciphering digital data sequences.
In its more particular aspects the present invention relates to a method of, and apparatus for, encoding or enciphering digital data sequences by transforming such data sequences into an encoded or enciphered data sequence under the supervision of a confidential or secret key or code for the protected transmission or storage of such digital data sequences. The digital data sequence is combined with a digital pseudo-random sequence produced by a converter or running-key generator.
Various encoding or enciphering systems are already known. Some of these systems are described, for example, in the textbook "CIPHER SYSTEMS, The Protection of Communications" by Henry Beker and Fred Piper, Northwood Publications London, 1982. In such systems, for example, messages or other data in binary code are combined with a pseudo-random sequence produced by a running-key generator under the supervision of a secret key to form the enciphered sequence that is, stored and/or transmitted and then decoded or deciphered. The knowledge of a confidential or secret code or key is obviously necessary for the decoding or deciphering operation. The synchronizing methods for transmitting the encoded or enciphered data and for its decoding or deciphering are also known and are also described in the aforementioned textbook. In this textbook, a converter or running-key generator for generating or producing pseudo-random sequences is described and illustrated on Page 237, FIG. 6.8. This converter or running-key generator operates using a number of feedback-connected shift registers whose periodic digital output sequences are combined. This method of combining the periodic digital sequences, however, utilizes neither memory nor feedback and thus the effectiveness of the encoding or enciphering is limited.